THE EARLY TRIASSIC `LIZARD' COLUBRIFER CAMPI: A REASSESSMENT by
SUSAN E. EVANS
ABSTRACT. Colubrifer campi Carroll, 1982 is a small reptile represented by a single skeleton from the Lower Triassic of South Africa. Carroll attributed the genus to the Squamata, proposing that it was a derived lizard with reduced limbs. Re-examination of the specimen permits a rather different interpretation of the skull. The holotype of Colubrifer is the skeleton of a small primitive procolophonian, almost certainly referable to the established South African genus Owenetta. KEY WORDS:
Triassic, lizards, procolophonians, South Africa.
I N 1982, Carroll described a small fossil reptile skeleton from the Lystrosaurus Assemblage Zone of South Africa, referring it to a new genus and species of fossil lizard, Colubrifer campi. The skull was interpreted as being well advanced towards the squamate condition, while the relatively short limbs and elongated axial skeleton led the author to suggest that Colubrifer was an advanced lizard either paralleling, or ancestral to, modern squamates with reduced limbs. Since limb reduction does not occur in iguanians or more basal squamate taxa, this would imply that Colubrifer was a scleroglossan lizard, and that crowngroup squamates had diversi®ed into their major clades by the end of the Permian. By contrast, Estes (1983) removed Colubrifer from the Squamata due to the absence of an external quadrate conch, but described it as `one more example of modernization towards the lizard condition within Triassic reptiles' (1983, p. 209). The type and only specimen (Museum of Paleontology, University of California, Berkeley, UCMP 42773) is represented by an incomplete skull and postcranial skeleton, both preserved in ventral view. Carroll initiated further preparation to expose the dorsal surface of the skull and his identi®cation is based primarily on the interpretation of this dorsal view. The skull is incomplete and the bones are partly disarticulated, causing some confusion. Re-examination suggests a radically different interpretation of the skull morphology. The skull is not that of a diapsid, but rather that of a small primitive procolophonian. THE MORPHOLOGY OF THE SKULL
The palate (Text-®g. 1A) The interpretation of the palate is not controversial, but this view does yield important information for the understanding of the dorsal surface. The most clearly preserved elements are a large right pterygoid and part of the right lower jaw. The pterygoid has a broad palatal plate, a deep basipterygoid notch, and a short deep quadrate process. The palatal plate bears two principal tooth rows, one running along the medial border of the bone, the other running across the pterygoid at about 45 degrees to the ®rst. From here, the second row would have continued onto the palatine. The position of this row suggests that the palatine extended well posteriorly. The pterygoid ¯ange is broad and directed anterolaterally. The presence or absence of a pterygoid ¯ange tooth row cannot be determined (contra Carroll 1982) because the bone surface along the ridge has been lost. A fragment of bone in articulation with the tip of the pterygoid ¯ange must be part of the ectopterygoid. In front of this is what appears to be a slight emargination, but the edge is obscured by matrix (shown by the dotted line in Text-®gure 1A), and there is no evidence of a suborbital [Palaeontology, Vol. 44, Part 5, 2001, pp. 1033±1041]
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TEXT-FIG
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1. `Colubrifer campi' Carroll 1982 (UCMP 42773). A, skull in palatal view, and B, right lower jaw, lateral view. Hatching indicates broken surfaces. Scale bar represents 1 mm.
fenestra. The edge of the bone between the pterygoid ¯ange and the palatine tooth row probably carried facets for ectopterygoid and palatine. The irregular element to the right (topographically) of the basipterygoid notch has been interpreted as part of the parabasisphenoid (Carroll 1982, ®g. 2B), and this seems likely, but the other identi®cations cannot be con®rmed. The large elements to the right (topographically) and deep to the pterygoid are parietals rather than the left pterygoid (see below). The right lower jaw lies in anatomical position along the right margin of the palate. Its structure is not clearly preserved, but the dentary bears at least 22 small, slender teeth, with slightly recurved tips and a rounded cross-section (Text-®g. 1B). The dorsal surface of the skull (Text-®g. 2) Text-®gure 2A shows a view of the dorsal surface of the skull as currently preserved, while Text-®gure 2C shows the original interpretation of the elements. The key identi®cation was of the two roughly square bones in the centre of view as parietals (Carroll 1982, ®g. 2A; Text-®g. 2C). The bones are of similar size and shape, and the identi®cation was a logical one. The midparietal suture thus became the axis of
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TEXT-FIG. 2. `Colubrifer campi' Carroll 1982 (UCMP 42773). Dorsal view of the skull. A, as preserved; B, as interpreted in this paper; C, as interpreted by Carroll (1982, ®g. 2A). Scale bar represents 1 mm.
symmetry around which the remaining skull bones were identi®ed, using a diapsid skull as a model. Re-examination of the dorsal surface should, therefore, begin with these `parietal' elements. Despite their obvious similarities, the two bones actually show differences in the depth and structure of their lateral and posterior borders, and in the shapes and relations of their anterior borders. What was originally interpreted as the right parietal (Text-®g. 2A, C) has a nearly straight, but facetted anterior border and its thickened and notched lateral border probably contributed to the edge of the skull table. However, the element to its immediate left has an anterior border that extends forward under the bone in front. The shape is dif®cult to see in dorsal view without reorienting the specimen, but is clearly visible from the palatal surface (Text-®g. 1A: P). Furthermore, the `lateral' edge of this `left parietal' is thin, ¯attened, and emarginated anteriorly. Much of its `lateral' edge is overlapped by a matching, but incomplete element that was not identi®ed in the original description although it appears to have been larger at that time (Text-®g. 2C: ?). Closer examination of the skull in both views shows that there is an articulation between these two bones. Anterior and posterior to the `parietals', the symmetry breaks down. The presumed left and right frontals (Text-®g. 2C: lF and rF) differ in their shape, and their sutural relations to other bones are not the same. Posteriorly, the axis of symmetry of the braincase is not aligned with that of the conjectured skull midline. These problems are resolved if the skull midline is repositioned one element to the left, so that it passes between Carroll's `left parietal' and its immediate unidenti®ed left hand neighbour (Text-®g. 2B). Then the large notch which partially separates these bones is the parietal foramen, and the elongated bone which overlaps the true right parietal is the right frontal (Carroll's left). The shape of the posterior end of this right frontal matches the shape of the facet on the parietal below it. Carroll's `right parietal' is actually the right supratemporal, and the two bones in front of this supratemporal, between it and the right frontal, are the right postorbital (original postfrontal) laterally, and right postfrontal (original right frontal) medially. The original `nasals' are probably parts of the right prefrontal and right maxilla, while the right `postorbital or
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jugal' is probably the squamosal, and the right quadrate, either the quadrate or quadratojugal. Thus the specimen actually preserves only the right side of a broad, anapsid skull roof, with a large, laterally placed supratemporal and a large parietal foramen. POSTCRANIAL SKELETON
The postcranial skeleton is only partially preserved (Text-速g. 3A). For Carroll (1982), the most signi速cant feature was the comparatively long vertebral column relative to the length of the limbs. This led to the suggestion that Colubrifer might be ancestral to one lineage of living reduced-limbed squamates. He also judged that the ankle, humerus and scapulocoracoid showed derived lepidosaurian or squamate features. Carroll's (1982) count of `at least' 27 presacral vertebrae appears accurate, and the axial skeleton is indeed relatively long. Only the cervical vertebrae are preserved in any detail. There are around seven in total, with centra bearing strong ventral crests and separated by broad intercentra rather than squamate hypapophyses. Where visible, the cervical ribs are two-headed, while the dorsal ribs appear single-headed. The vertebrae at the end of the presacral series are quite short and have large arching zygapophyses. Nothing is visible of the sacrum.
3. The skeleton of A, Colubrifer campi (UCMP 42773); and B, Owenetta rubidgei. A, redrawn from Carroll (1982, 速g. 1); B, partially redrawn from Reisz and Laurin (1992, 速g. 1). Scale bars represent 10 mm.
TEXT-FIG.
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Pectoral girdle and forelimb Parts of the clavicles, interclavicle, scapulae and coracoids are probably present, but the preservation is poor. The scapula blade may be quite tall but (contra Carroll 1982), there is no evidence of either emargination or notching. The humerus is gracile, with a comparatively long shaft and narrow proximal and distal heads. The presence of an ectepicondylar foramen or groove remains uncon®rmed, but the entepicondylar foramen does indeed appear to be absent (Carroll 1982). Pelvic girdle and hind limb The pelvic elements are separated and clearly visible. The ilium seems to have a vertical rather than inclined blade and there was evidently no thyroid fenestra between the pubis and ischium. The femur is strongly built and slightly sigmoid. Distal parts of the crural bones are preserved in association with the proximal tarsal mass. Whether the astragalus and calcaneum are actually fused is unclear, but they do appear to have formed a functional unit (Carroll 1982). DISCUSSION
The relationships of Colubrifer The structure of the skull as interpreted here suggests that Colubrifer was not a diapsid, much less a lepidosaur or a derived early lizard. There are no upper temporal fenestrae and no obvious suborbital fenestrae. In the Early Triassic, the most common small anapsid reptiles were the parareptilian procolophonians, and these provide the closest match for the skull of Colubrifer (Text-®g. 4). In the palate, the broad pterygoids with reduced tooth rows, a strong basipterygoid notch, and the anterolaterally directed transverse pterygoid ¯ange are all features found in procolophonians (e.g. De Braga and Rieppel 1997). The status of the pterygoid ¯ange tooth row remains unclear in Colubrifer, but loss of this row is a character of procolophonians as well as lepidosaurs. In dorsal view, the short, wide parietals extending forward in the midline, the large parietal foramen, and the enlarged lateral supratemporals are also procolophonian features. Nothing in the postcranial skeleton contradicts this conclusion. Details of the pectoral girdle are unclear, but a tall scapula is as much a procolophonian character as a lepidosaurian one. However, whereas lepidosaurs are characterised by the presence of a thyroid fenestra in the pelvis, this is absent in Colubrifer as in all primitive reptiles, including parareptiles. Although Sphenodon and chameleons have an unusual vertical iliac blade, this was not the condition in basal rhynchocephalians like Gephyrosaurus (Evans 1981), and a tapering, obliquely oriented blade appears to have been the primitive lepidosaurian condition. A short vertical blade is, however, a feature of parareptiles (DeBraga and Rieppel 1997). Similarly, a presacral vertebral count of 24±26 is typical for procolophonians as well as lepidosaurs, with a higher count in some derived squamates but also in some primitive procolophonians (see below). Two-headed cervical ribs are found in procolophonians but not squamates (dibamids being a highly derived exception). Slender limb bones with relatively long shafts and narrow distal ends are also found in some procolophonians, and procolophonians may fuse the astragalus and calcaneum (Romer 1956; De Braga and Rieppel 1997), paralleling the condition in lepidosaurs. Like derived lepidosaurs, many procolophonians lose the entepicondylar foramen (Reisz and Laurin 1991). Thus the structure of the skull supports the conclusion that Colubrifer is a procolophonian rather than a diapsid, and what is known of the postcranial skeleton is consistent with this. The recent classi®cation of De Braga and Rieppel (1997) recognised two families of procolophonians: Procolophonidae and Owenettidae. Procolophonids from the Early Triassic onwards (e.g. Procolophon, Kapes, Burtensia) show a derived morphology with a reduced number of enlarged cheek teeth (Text-®g. 4A±C). This is in contrast to the many slender teeth in Colubrifer. There are also striking differences between typical Triassic procolophonids and Colubrifer in the orbital region, where the postfrontal of derived forms is small and the parietal enters the posterior orbital margin (Text-®g. 4A), and in the shape of the supratemporal which is extended posteriorly in procolophonids but not Colubrifer (Carroll and Lindsay 1985). Colubrifer cannot therefore be referred to the Procolophonidae.
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4. The procolophonian skull. A±C, the skull of Procolophon trigoniceps in A, dorsal; B, palatal; and C, lateral views (redrawn from Carroll and Lindsay 1985, ®g. 1); D±F, the skull of Owenetta rubidgei in A, dorsal; B, palatal; and C, lateral views (redrawn from Reisz and Laurin 1991, ®g. 2). Scale bars represent 10 mm.
TEXT-FIG.
The smaller family Owenettidae includes Barasaurus from Madagascar and Owenetta from South Africa. Comparison between Colubrifer and Owenetta shows clear similarities (Text-®gs 2B, 3, 4D±F). The two genera share primitive characters like the many small, sharp, slightly recurved teeth on the dentary, and the large postfrontal and postorbital completing the dorsal orbital margin. They also share the square supratemporal lying lateral to the parietal, and the pattern of pterygoid teeth. Postcranially, Owenetta has a long vertebral column with more than 27 presacral vertebrae, short vertebral bodies, and expanded zygapophyses (Text-®g. 3B). The limbs are gracile and have narrow proximal and distal ends. Since the presacral column is long, the limbs appear relatively short. Thus the body proportions of Owenetta match those of Colubrifer without the need to infer specialisation towards limb-loss (Text-®g. 3). Carroll (1982) recorded Colubrifer as coming from the Lystrosaurus Assemblage Zone at the locality of Thaba Nchu, although according to Kitching (1977) both Lystrosaurus Assemblage Zone and Dicynodon Assemblage Zone (Upper Permian) deposits are exposed there. Owenetta is a morphologically primitive procolophonian originally described from the Upper Permian of South Africa (Broom 1939; Gow 1977). It is comparatively rare, probably because of its gracile build (Gow 1977), but is known to have extended from the Cistecephalus Assemblage Zone (Owenetta rubidgei), through the Dicynodon Assemblage Zone (O. rubidgei ), and into the Lystrosaurus Assemblage Zone (Owenetta sp.) (Kitching 1977, 1995; Groenewald and Kitching 1995). In Dicynodon Assemblage Zone and Lystrosaurus Zone assemblages, Owenetta is found in association with very similar synapsids to those found at Thaba Nchu (Kitching 1977; Groenewald and Kitching 1995).
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CONCLUSION
Under the new interpretation presented above, the incomplete holotype of Colubrifer campi (UCMP 42773) closely resembles that of the procolophonian Owenetta in the morphology and proportions of both the skull and postcranial skeleton. The two taxa were also contemporaneous. Together, these observations support the conclusion that the type and only specimen of Colubrifer is an incompletely preserved specimen of Owenetta sp., and that the generic name Colubrifer Carroll, 1982 is a junior synonym of Owenetta Broom, 1939. Colubrifer was one of several small Permian and Triassic reptiles originally described as lizards (e.g. Robinson 1962, 1967; Carroll 1975, 1977; Carroll and Galton 1977; Tatarinov 1978; Bartholomai 1979). On re-examination, they have all been shown to lie outside Lepidosauria, either as offshoots from the lepidosauromorph or diapsid stem (Evans 1980, 1982, 1984, 1988; Gauthier et al. 1988) or as members of completely unrelated groups (Evans and Milner 1989; this paper). Despite recent molecular analyses suggesting otherwise (Hedges and Poling 1999), the overwhelming body of evidence supports the sister group relationship of Rhynchocephalia (Sphenodon and its fossil relatives) and squamates within a monophyletic Lepidosauria. Amongst reptiles, rhynchocephalians and squamates share a unique suite of skeletal and soft part character states including separate centres of ossi®cation on their long bones, specialised mesotarsal and ®fth metatarsal structures, a complex skin shedding/replacement cycle, the possession of hemipenes (albeit rudimentary in Sphenodon) and a transverse cloacal slit (Evans 1984, 1988; Gauthier et al. 1988). The earliest currently recorded rhynchocephalians come from the Upper Triassic (Carnian) of Britain (Brachyrhinodon) and Germany (Polysphenodon) (Fraser and Benton 1989). However, these genera are nested well within Sphenodontia (Fraser and Benton 1989; Wilkinson and Benton 1996), crownward of the basal Gephyrosaurus (Lower Jurassic, Britain; Evans 1980), Diphydontosaurus (Upper Triassic, Britain; Whiteside 1986), and Planocephalosaurus (Upper Triassic, Britain; Fraser 1982). This is consistent with a Middle Triassic radiation of early rhynchocephalians, and an Early±Middle Triassic split of the lepidosaurian stem into its two daughter lineages. Living squamates are divided into two major clades: Iguania (iguanids, agamids and chameleons) and Scleroglossa (scincomorphs, anguimorphs, gekkotans, amphisbaenians and snakes) (Estes et al. 1988). The earliest certain records of squamates are currently from the Middle Jurassic (Bathonian) of Britain (Evans 1993, 1994, 1998), but these Bathonian lizard assemblages already include a range of scleroglossans (scincomorphs, an anguimorph, and possible gekkotans), some of which (e.g. the anguimorph Parviraptor Evans, 1994) are members of derived groups. This provides compelling evidence that scleroglossan diversi®cation was well underway by the Middle Jurassic and that the iguanianscleroglossan dichotomy took place in the Late Triassic or earliest Jurassic, with basal squamates occurring even earlier. The squamate and rhynchocephalian records are therefore consistent with one another. Early lepidosauromorphs were small animals with delicate skeletons. Their preservation and subsequent recovery depends on suitable depositional environments and appropriate collecting techniques. Most small Mesozoic fossil skeletons still come either from ®ne-grained laminated limestones (e.g. Sanz et al. 1988) or from microvertebrate sites (e.g. Evans 1998), and relatively few of these are known for the Triassic. Furthermore, in Jurassic and Lower Cretaceous horizons, squamates are most often found in association with an essentially mesic assemblage including frogs, salamanders and other freshwater taxa (Evans 1995, 1998). Unfortunately, most known Triassic microsites represent arid upland environments rather than mesic ones (Czatkowice, see below, is an exception). When there are Triassic localities comparable to Kirtlington (Middle Jurassic, England; Evans and Milner 1994), Guimarota (Upper Jurassic, Portugal; Seiffert 1973), the Purbeck Limestone Formation (Lower Cretaceous, England; Ensom et al. 1987, 1994) and Las Hoyas (Lower Cretaceous, Spain; Sanz et al. 1988), the missing Triassic records of both squamates and lissamphibians may be recovered. This potential is shown by the Lower Triassic locality of Czatkowice, Poland (Borsuk-Bialynicka et al. 1999), where deposits representing locally mesic conditions have yielded a diversity of small reptiles, including a relatively derived lepidosauromorph (BorsukBialynicka et al. 1999), as well as the only Triassic stem-group frog outside Madagascar (Evans and Borsuk-Bialynicka 1998).
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Typescript received 19 May 2000 Revised typescript received 8 September 2000
Department of Anatomy and Developmental Biology University College London Gower Street London WC1E 6BT, UK
APPENDIX Abbreviations used in the text-®gures AC, astragalus/calcaneum; Br, braincase; Cr, crural bones (tibia/®bula); D, dentary; Ec, ectopterygoid; F, frontal; Fe, femur; H, humerus; Hd, hand; I, ilium; Ic, interclavicle; Is, ichium; J, jugal; lF, left frontal; lJ, lower jaw; lP, left parietal; ?Mx and Pfr, possible maxilla and prefrontal; N, nasal; P, parietal; Pa, palatine; Pa. f, parietal foramen; ?Pb, parabasisphenoid; Pe.g, pectoral girdle; Pf, postfrontal; Po, postorbital; Pt, pterygoid; Pu, pubis; Q, quadrate, ? possible quadrate; Q j, quadratojugal; rF, right frontal; rLJ, right lower jaw; ?rMx-Pfr, possible right maxilla and prefrontal; rN, right nasal; rP, right parietal; rPf, right postfrontal; rPo, right postorbital; rPo-J, right postorbital or jugal; rSt, right supratemporal; Sq, squamosal, ? possible squamosal; St, supratemporal.